Take installed packages into account when upgrading another package

[nlhkabu]

In #7744 we have decided to warn users when pip creates a dependency conflict when upgrading another package.

This issue is a proposal to change this behavior. This proposal is supported by user research, a summary of which will shortly be published on pip's docs.

What's the problem this feature will solve?
Currently, when a user asks pip to upgrade a package, it does this without considering what other packages are already installed. This can mean that pip can cause a dependency conflict.

This scenario (used in our user research) summarises the problem well:

Imagine you have package tea and coffee with the following dependencies:

tea 1.0.0 - depends on water<1.12
tea 2.0.0 - depends on water>=1.12
coffee 1.0.0 - depends on water<1.12
coffee 2.0.0 - depends on water>=1.12

You have the following packages installed:

tea 1.0.0
coffee 1.0.0
water 1.11.0

You ask pip to upgrade tea.  What should pip do?

If pip upgrades tea to 2.0.0, water needs to be upgraded as well, creating a conflict with coffee...

Describe the solution you'd like
Pip should:

• Detect whether or not a conflict may occur, based on the packages that are already installed
• If a conflict is detected, pip should not install anything, and show an error message to the user

This reflects the most popular response to our research (71.7% of respondents) who said that pip should "Install nothing. Show an error explaining that the upgrade would cause incompatibilities."

Additional context
Example error message (happy to iterate on this as needed!)

ERROR: pip cannot upgrade tea, as this would introduce a dependency conflict:
  you have water 1.11.0 installed
  you have coffee 1.0.0 installed, requires water<1.12
  tea 2.0.0 requires water>=1.12

[pradyunsg]

Awesome. Looks like a supermajority of the folks we've asked in the survey about "should pip take installed packages into account" responded that we should. We'd likely want to do this at a later date, sometime in 2021.

IF @pypa/pip-committers (specifically, @pfmoore or @uranusjr) want to try to make this happen sooner, I'm not opposed. ;)

[junpuf]

@nlhkabu

It looks like I didn't catch the bus when the user survey was on going. But I also agree with "Install nothing. Show an error explaining that the upgrade would cause incompatibilities.". But I do have one question regarding this.

Will we be just getting a warning that has ERROR text in it? Or are we getting a non-zero exit code as well?

[uranusjr]

I’d expect the command to exit with non-zero since the message says cannot.

[miguelbalmeida]

Just adding my +1 here in favor of the same option as the 71.7% in your survey. I prefer that pip does not install anything and exits (with non-zero exit code) if proceeding would create an inconsistent environment.

@nlhkabu , +1 on your suggested error message also.